1. Field of the Invention
The present invention relates to nickel powder, and more particularly, to composite nickel powder.
2. Description of the Related Art
Nickel powder has various uses. One of the representative uses is the one as a material for manufacturing an inner electrode of an MLCC (multi-layer ceramic capacitor).
Generally, an MLCC is manufactured by laminating a number of thin dielectric layers and a number of inner electrodes. Although the MLCC has a relatively small volume, it has a large accumulating capacitance. Thus, the MLCC is widely used in electronic devices such as a computer, a mobile communication device, etc
Ag—Pd alloy is used as an inner electrode material of MLCC. Ag—Pd alloy can be easily applied to the manufacturing of the MLCC because it is fired (sintered) in the air, but it is expensive. There was a tendency of replacing the inner electrode material with nickel, which is inexpensive, to lower the cost of the MLCC in the late 1990's. The nickel inner electrode of the MLCC is formed by coating a conductive paste, which comprises nickel metal powder, drying and co-firing.
It is required to minimize the size of an electronic component, particularly the MLCC, in order to continuously minimize the size of an electronic device. To minimize the size of the MLCC, ultra-thin ceramic dielectric layers and inner electrode layers are required.
Generally, an MLCC is manufactured by co-firing the ceramic dielectric layers and the inner electrode layers. A shrinkage rate of the inner electrode layer is higher than that of the ceramic dielectric layer because the inner electrode layer, before firing, has a low packing density with a high content of organic vehicles. Also, the temperature of shrinking of the nickel is about 400 to about 500° C., whereas that of BaTiO3 generally used as the ceramic dielectric layer is more than about 1,100° C. These differences of the shrinkage rate and the temperature of shrinking between the inner electrode layer and the ceramic dielectric layer lead to a lowering of the connectedness of the inner electrode and delamination.
In order to lower the shrinkage rate and increase the temperature of shrinking of the nickel powder, it is proposed to reduce the oxygen content of the nickel powder and to use a composite nickel powder such as oxide-coated nickel powder. MgO, SiO2, TiO2, BaTiO3, oxides of the rare-earth elements, etc. were used as nickel powder coating oxides. “Dry-type mechanochemical mixing” using a hybridizer (see the Japan patent laid-open publication No. 1999-343501), “Spray pyrolysis” (see U.S. Pat. No. 6,007,743), and “Wet-type sol-gel coating” (see the Japan patent laid-open publication No. 2002-25847) were used to coat the nickel powder with oxides.
In the case of the oxide-coated nickel powder manufactured by the mechanochemical mixing, bondage between the oxide particles and nickel particles is weak, and when it is processed into paste, there is a possibility that the oxide-coated nickel powder will be divided into the oxide particles and the nickel particles. Moreover, the improved effect of the heat shrinkage rate of the oxide-coated nickel powder manufactured by the mechanochemical mixing is known to be very low (see the Japan patent laid-open publication No. 1999-343501).
In the case of the above mentioned spray pyrolysis, the nickel powder comprising composite oxide was prepared by spraying the solution comprising a precursor of nickel and a thermally decomposable compound which can form a coating layer, and thermal decomposing. In the case of the nickel powder manufactured by the above mentioned spray pyrolysis, however, oxides were formed not only on the surface of the nickel particle, but also in the nickel particle. Due to this, the oxides can remain as impurities after forming the nickel electrode (see the U.S. Pat. No. 6,007,743).
In the case of the wet-type sol-gel coating, the physico-chemical coating is carried out by adding the nickel powder into an aqueous solution of coating layer-forming material and reacting the solution with the nickel powder. And then, the coating layer of the coated nickel powder is crystallized by heat treatment of the coated nickel powder. Compared to the oxide-coated nickel powder manufactured by the mechanochemical mixing, the oxide-coated nickel powder manufactured by the above mentioned wet-type sol-gel coating has stronger bonding powder to the coating layer. Also, unlike the oxide-coated nickel powder manufactured by the spray pyrolysis, the oxide-coated nickel powder manufactured by the wet-type sol-gel coating has an oxide layer with the desired content only on its own surface.
However, since most wet-type sol-gel coating methods use water-based coating solution (see the Japan patent laid-open publication No. 2001-131602), hydroxyl groups remain in the coating layer of the produced nickel powder. During the process of drying, agglomeration of the oxide-coated nickel powder occurs by a condensation reaction of the remaining hydroxyl groups. The agglomerates formed during the drying process are maintained as formed during the heat treatment process for crystallization, and the strength of the agglomerates increases as the crystallization of the coating layer is increased.
Conductive paste is manufactured by dispersing an oxide-coated nickel powder in an organic solvent, and the conductive paste is printed on a dielectric sheet, thereby forming an inner electrode layer. The properties of the inner electrode layer printed on the dielectric sheet can be fatally affected by the agglomeration of the nickel powder in the conductive paste. That is, the agglomerated nickel powder protruded from the inner electrode layer and the roughness of the inner electrode layer was increased. When the inner electrode layer having increased roughness is fired, a breaking of the inner electrode layer occurred so that the quality of the MLCC is lowered.